I mean I think it can be boiled down pretty simply: cause the least harm to living things that you can personally manage, according to your definition of harm. Having impossible goals isn't necessarily a bad thing. If your impossible goal is to make a billion dollars ethically, and you get to 50 million being 95% ethical, you could still consider that a win, even though you didn't reach your impossible goal.

Even the simple goal of "always being a good person 100% of the time" is probably impossible to achieve over an entire lifetime while meeting every person's definition of it. That doesn't mean it's useless for someone to strive for that within their definition of "good person".

In fact I'd say the vast majority of meaningful, non trivial goals could be considered "impossible".